1. Field of the Invention
This invention relates to a method for producing a grain-oriented electrical steel sheet used mainly for iron cores of transformers and the like.
2. Description of the Related Art
Various technologies have been proposed for stably producing a grain-oriented electrical steel sheet having excellent magnetic properties with a magnetic flux density B8 (magnetic flux density in a magnetic field of 800 A/m) exceeding 1.9 T. The technologies can be classified, generally, into the following three groups.
The first group of technologies consists of a method of heating a slab to an ultra high temperature of 1,350 to 1,450xc2x0 C., at the maximum, and retaining the slab at the heating temperature for a period of time sufficient for heating (soaking) the entire slab. The object of this method is to change substances uniformly acting as inhibitors, such as MnS, AlN, etc., into complete solutions in order to activate them as inhibitors necessary for secondary recrystallization. Since the complete solution heat treatment is also effective as a measure to eliminate a difference in the intensity of the inhibitors in different parts of a slab, the above method is reasonable in this respect for realizing stable production of the products.
In the above method, however, the heating temperature necessary for the complete solution of substances having the inhibition capacity, or the complete solution temperature, is very high. Since the slab has to be heated, in actual production practices, to a temperature equal to or above the complete solution temperature (an ultra high temperature) in order to secure the amounts of inhibitors necessary for secondary recrystallization, the method involves various problems in actual production practice.
The problems include, for example, the following: {circle around (1)} it is difficult to secure a desired rolling temperature during hot rolling and, when the desired temperature is not achieved, poor secondary recrystallization occurs because inhibitor intensity becomes uneven in a slab; {circle around (2)} coarse grains form during the heating for hot rolling and the portions with the coarse grains fail to re-crystallize at the secondary recrystallization, leading to streaks; {circle around (3)} slab surface layers melt into slag, which in turn requires a large amount of manpower for the maintenance of reheating furnaces; and {circle around (4)} product yield decreases because huge edge cracks occur in the hot rolled steel strips.
As improvements to the first group technologies, methods intended to stabilize the secondary recrystallization by applying a nitriding treatment after primary recrystallization based on the above method are known, such as those disclosed in Japanese Unexamined Patent Publication No. H1-168817, etc. The problem this method can solve, however, is only the one described in {circle around (1)} above, and the solution of the problems in field production practices described in {circle around (2)} to {circle around (4)} above still remains difficult.
The second group of technologies combine the use of AlN as an inhibitor, heating of a slab to below 1,280xc2x0 C. and a nitriding treatment after a decarburization annealing and before the commencement of the secondary recrystallization, as disclosed in Japanese Unexamined Patent Publications Nos. S59-56522, H5-112827 and H9-118964, etc. In a method like the above, it is very important, to obtain a satisfactory secondary recrystallization, to control the mean size of primary recrystallization grains after the decarburization annealing within a prescribed range, usually 18 to 35 xcexcm, as shown, for example, in Japanese Unexamined Patent Publication No. H2-182866.
Besides the above, Japanese Unexamined Patent Publication No. H5-295443 discloses a method to control the steel composition, etc., in order to minimize solute nitrogen, etc., at heating during hot rolling, for the purpose of homogenizing the size of the primary recrystallization grains in a coil, based on the fact that the solid solution amount in steel of the substances having the inhibition capacity such as solute nitrogen at heating during hot rolling, etc., determines the growth of the primary recrystallization grains.
By this method, although, however accurately controlled the steel composition may be, uneven distribution of the solute nitrogen, etc., remains in a slab, and it is impossible to eliminate, in the strict meaning of the word, the uneven distribution of the inhibition intensity, or that of the primary recrystallization grain size, within a coil. This results in a problem that it is sometimes difficult to obtain homogeneous secondary recrystallization within a coil (skid mark). Thus the above method is not an industrially stable production method.
The third group of technologies consist of a method to use CuxS (x=1.8 or 2) as an inhibitor and heat a slab to a temperature equal to or above the complete solution temperature of CuxS and equal to or below the complete solution temperature of MnS, as disclosed in Japanese Unexamined Patent Publication No. H6-322443, etc. The characteristics of this method lie in lowering the slab heating temperature and making additional process steps, such as the nitriding treatment employed in the second group of technologies, unnecessary.
This method, however, has a problem similar to the one involved in the second group of technologies (skid mark), because the slab heating temperature is equal to or below the complete solution temperature of MnS, and thus it is not an industrially stable production method, either. Besides, although CuxS is widely known as an inhibitor to control the secondary recrystallization, it is inappropriate for the production of a grain-oriented electrical steel sheet having high magnetic flux density especially when a final cold rolling reduction ratio exceeds 80% (Tetsu-to-Haganxc3xa9, p. 2049, No. 15, Vol. 70, 1984).
Generally speaking, whether or not it is possible to obtain a secondary recrystallization having good magnetic properties is determined mainly by the grain diameter of the primary recrystallization and secondary inhibitors to control the secondary recrystallization. While the grain diameter of the primary recrystallization by the first group technologies is about 10 xcexcm, for example, the same by the second group technologies is 18 to 35 xcexcm. The fact that it is possible to obtain a good secondary recrystallization by either the first or the second group of technologies, in spite of the fact that the diameter of the primary recrystallization grains is greatly different by the two groups of technologies as the examples above, indicates that the combination of the grain diameter of the primary recrystallization and the secondary inhibitors necessary to obtain a sharp Goss (the {110} less than 001 greater than  orientations) secondary recrystallization, is not unique.
In view of the above fact, the present inventors carried out a series of studies based on an idea that it was possible to obtain a sharp Goss secondary recrystallization by controlling the secondary inhibitors, regardless of the size of the primary recrystallization grains.
For the purpose of establishing a method to stably produce the product under the above facts, the present inventors classified the inhibitors indispensable for the production of a grain-oriented electrical steel sheet, by the process step where they function, into two groups, namely primary inhibitors to control the size of the primary recrystallization grains and secondary inhibitors to control that of the secondary recrystallization grains, and studied them in relation to the production of a grain-oriented electrical steel sheet having excellent magnetic properties.
It has to be noted here that, although it is true that the combination of the primary recrystallization grain size and the secondary inhibitors necessary, for obtaining a sharp Goss secondary recrystallization, is not uniquely defined, if the primary recrystallization grain size is different in different parts of a slab (coil), for example, a good orientation of the secondary recrystallization grains cannot be obtained unless the intensity of the secondary inhibitors is appropriately controlled in different portions of a coil. For this reason, a stable production method is defined as the one to provide a homogeneous grain size throughout the entire coil at both the primary and the secondary recrystallization.
It is also desirable that the intensity of the primary inhibitors be uniformly distributed throughout the entire slab, since the primary recrystallized grain size is determined by the intensity of the primary inhibitors and the temperature of a decarburization annealing during which the primary recrystallization takes place.
The most important point for establishing a stable production method of the product is, therefore, how to uniformly distribute both the primary and secondary inhibitors throughout a coil.
In this respect, the above first to third groups of technologies have the following problems, respectively:
In the first group technologies, it is very difficult to secure the inhibitor intensity necessary for the secondary recrystallization and, at the same time, realize stable product quality in an industrial production scale because, according to the technologies, it is necessary to heat a slab within an extremely narrow temperature range, namely the complete solution temperature of inhibitors or higher and a temperature below the temperature of forming coarse grains during the heating in hot rolling at which the secondary recrystallization becomes unstable without xe2x80x9cpre-rolling process (break down)xe2x80x9d.
In the second group technologies, it is easy to secure the intensity of the secondary inhibitors by applying a nitriding treatment after the decarburization annealing and before the secondary recrystallization during final box annealing but, when viewed from the standpoint of the homogeneity of the primary inhibitor intensity, finite amounts of solute nitrogen and the like are distributed unevenly in different portions of a slab (coil), and this results in uneven grain size of the primary recrystallization grains. Further, in this case, the uneven distribution of the primary inhibitors within an entire slab (coil) leads to an uneven distribution of the secondary inhibitors, too, since the primary inhibitors function also as the secondary inhibitors.
The third group technologies are disadvantageous, similar to the second group technologies, in terms of uniform distribution of the primary inhibitors within a slab (coil), since no heat treatment is applied for complete solution of MnS, and 60% or more of AlN is made to precipitate after hot rolling. In the technologies, the secondary inhibitors are not changed from the primary inhibitors because no inhibitor intensifying treatment has been applied at any intermediate process and thus the secondary inhibitors are unevenly distributed in different portions of a coil. As a consequence, it is difficult by these technologies to secure stable product quality industrially. In addition, as explained before, although CuxS is widely known as an inhibitor to control the secondary recrystallization, it is inappropriate for the production of a grain-oriented electrical steel sheet having high magnetic flux density especially with a final cold rolling reduction ratio exceeding 80%.
The object of the present invention, which was worked out in view of the above background, is to provide a method capable of very stably producing a grain-oriented electrical steel sheet having excellent magnetic properties by making the secondary recrystallization yet more complete.
The gist of the present invention is as described in (1) to (8) below.
(1) A method for producing a grain-oriented electrical steel sheet excellent in magnetic properties comprising the steps of; heating a slab containing a prescribed amount of Al to a temperature of 1,200xc2x0 C. or higher, hot-rolling the slab into a hot rolled strip, optionally annealing the hot rolled strip, cold-rolling the hot rolled strip, in one stage or in two or more stages with intermediate annealing(s), and decarburization annealing the cold rolled sheet and final box annealing after the application of an annealing separator to prevent strip sticking during the annealing, characterized by heating the slab to a temperature (slab heating temperature Ts (xc2x0 C.)) higher than the complete solution temperature of substances having capacities as inhibitors, and nitriding treating the decarburization annealed steel sheet before the commencement of secondary recrystallization during the final box annealing.
(2) A method for producing a grain-oriented electrical steel sheet excellent in magnetic properties according to item (1), characterized by heating the slab to a temperature of 1,350xc2x0 C. or lower.
(3) A method for producing a grain-oriented electrical steel sheet excellent in magnetic properties according to item (1) or (2), characterized by using a slab comprising, in mass %:
0.025 to 0.10% of C,
2.5 to 4.0% of Si,
0.01 to 0.10% of acid-soluble Al (sAl),
0.0075% or less of N,
0.003 to 0.05% of Seq (=S+0.406xc3x97Se), and
0.02 to 0.20% of Mn, and
the balance consisting of Fe and unavoidable impurities, and heating the slab to a slab heating temperature Ts (xc2x0 C.) higher than any of T1 (xc2x0 C.), T2 (xc2x0 C.) and T3 (xc2x0 C.) defined by the following equations, respectively, where indicates the mass % of the component element written[ ] inside the [ ];
T1=10,062/(2.72xe2x88x92log([sAl]*[N]))xe2x88x92273
T2=14,855/(6.82xe2x88x92log([Mn]*[S]))xe2x88x92273
T3=10,733/(4.08xe2x88x92log([Mn]*[Se]))xe2x88x92273.
(4) A method for producing a grain-oriented electrical steel sheet excellent in magnetic properties according to any one of items (1) to (3), characterized by using the slab comprising, additionally, 0.01 to 0.30 mass % of Cu, and heating the slab to a slab heating temperature Ts (xc2x0 C.) higher than T4 (xc2x0 C.) defined by the following equation, where [ ] indicates the mass % of the component element written inside the [ ];
T4=43,091/(25.09xe2x88x92log([Cu]*[Cu]*[S]))xe2x88x92273.
(5) A method for producing a grain-oriented electrical steel sheet excellent in magnetic properties according to any one of items (1) to (4), characterized by using the slab comprising, additionally, 0.0005 to 0.0060 mass % of B, and heating the slab to a slab heating temperature Ts (xc2x0 C.) higher than T5 (xc2x0 C.) defined by the following equation, where [ ] indicates the mass % of the component element written inside the [ ];
xe2x80x83T5=13,680/(4.63xe2x88x92log([B]*[N]))xe2x88x92273.
(6) A method for producing a grain-oriented electrical steel sheet excellent in magnetic properties according to any one of items (1) to (5), characterized in that the mean diameter of primary recrystallization grains after the decarburization annealing is 7 xcexcm or more and below 18 xcexcm.
(7) A method for producing a grain-oriented electrical steel sheet excellent in magnetic properties according to any one of items (1) to (6), characterized by controlling the increment of nitrogen in the steel sheet to 0.001 to 0.03 mass % by applying the nitriding treatment to the steel strip while it is running in an atmosphere of a mixed gas of hydrogen, nitrogen and ammonia.
(8) A method for producing a grain-oriented electrical steel sheet excellent in magnetic properties according to any one of items (1) to (7), characterized by controlling the cold rolling reduction ratio at the final cold rolling before the decarburization annealing to 80% or more and 95% or less.